1. Field of the Invention
The present patent application relates to a sheet size detection device and method for detecting multiple sheet sizes, and an image forming apparatus, such as a multifunctional machine including at least one of a copier, a printer, a facsimile machine, and a plotter, that includes the sheet size detection device and employs the sheet detection method.
2. Discussion of the Background Art
According to a background technique, a machine is caused to recognize the sizes of recording sheets stacked on a tray thereof on the basis of a manual operation performed by an operator. In the event of an operating error on the part of the operator, however, the machine cannot check the error. As a result, a failure such as a paper jam occurs.
Even in a machine provided with a tray capable of detecting the sheet sizes, if the detection is based on a method of detecting the sheet sizes only in a sheet conveyance direction, the detectable sheet sizes are limited. Meanwhile, there are mechanisms which detect the sheet sizes both in the sheet conveyance direction and a sheet width direction. Such mechanisms, however, only provide a general indication of the sheet sizes. Consequently, the detectable sheet sizes are limited, or the costs are increased due to multiple sensors provided to precision detection of the sheet sizes.
In the event of an error in the detection of a sheet size, even if a paper jam does not occur, an image forming unit performs such processes as the transfer of a toner image and the cleaning of toner remaining after the transfer process, in an area larger than the area of a recording sheet. As a result, a process cartridge is consumed more quickly and thus replaced more frequently. In other words, the life of the process cartridge is shortened.
In the control of a sheet feeding and separating unit, a situation is assumed in which the sheet size is unknown or the sheet size has been erroneously set. Therefore, it is necessary to bring forward the timing of stopping the drive of a feed roller included in the sheet feeding and separating unit. As a result, a sheet slip suppressing area is reduced, and the accuracy of sheet conveyance is degraded. Such deterioration affects productivity.
There are known sheet size detection devices for preventing the above-described undesirable phenomena attributable to operator error. Each of the background devices includes side fences and an end fence each slidable and positioned according to the sheet size, two levers interlocking with the side fences and the end fence, respectively, and a detection sensor including multiple push switch elements.
Each of the levers includes a concavo-convex pattern used for the recognition of the sheet sizes. The levers are superimposed to have the same center of rotation and the same locus formed by the leading end of the concavo-convex pattern away from the center of rotation. The thus-superimposed concavo-convex patterns form a combined concavo-convex pattern for selectively pressing the multiple push switch elements. With this configuration, multiple sheet sizes can be detected.
Along with the diversification of sheet sizes in the market, however, the possibility of erroneous recognition of the sheet sizes has been increasing, and a more minute concavo-convex pattern of the levers has been increasingly required. Further, greater accuracy of the operation performed by the operator is also required.
In an area of the concavo-convex pattern in which adjacent convex portions are closely located near each other, even a slight positional displacement of the side fences and the end fence can cause a change or a shift in the overall combined concavo-convex pattern. As a result, there is a risk of erroneous recognition of the sheet size.
Therefore, each of the fences needs to be accurately set at the position for the intended sheet size. Further, accuracy is constantly required in the sheet setting operation performed by the operator. In view of the above issue, it is conceivable to lower the detection accuracy by increasing the distance between the adjacent convex portions of the concavo-convex pattern. In such a case, however, the number of detectable sheet sizes is reduced.
According to one known sheet size detection device, a sheet feeding tray includes a member for locking, at a determined position, each of the fences serving as regulating members. However, it is easily predictable that the operator may forget to perform the locking operation.
If the operator forgets to perform the locking operation, the possibility of erroneous detection caused by the positional displacement of the fences is increased due to such factors as the shock caused in the attachment of the sheet feeding tray to the body of an image forming apparatus, the vibration occurring in an image forming operation, and the shock caused by unexpected external force exerted on the sheet feeding tray or the body of the image forming apparatus.